RFC 6052

IPv6 Addressing of IPv4/IPv6 Translators

Internet Engineering Task Force (IETF) C. Bao
Request for Comments: 6052 CERNET Center/Tsinghua University
Updates: 4291 C. Huitema
Category: Standards Track Microsoft Corporation
ISSN: 2070-1721 M. Bagnulo
UC3M
M. Boucadair
France Telecom
X. Li
CERNET Center/Tsinghua University
October 2010 IPv6 Addressing of IPv4/IPv6 Translators
Abstract
This document discusses the algorithmic translation of an IPv6
address to a corresponding IPv4 address, and vice versa, using only
statically configured information. It defines a well-known prefix
for use in algorithmic translations, while allowing organizations to
also use network-specific prefixes when appropriate. Algorithmic
translation is used in IPv4/IPv6 translators, as well as other types
of proxies and gateways (e.g., for DNS) used in IPv4/IPv6 scenarios.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6052.

1. Introduction
This document is part of a series of IPv4/IPv6 translation documents.
A framework for IPv4/IPv6 translation is discussed in
[v4v6-FRAMEWORK], including a taxonomy of scenarios that will be used
in this document. Other documents specify the behavior of various
types of translators and gateways, including mechanisms for
translating between IP headers and other types of messages that
include IP addresses. This document specifies how an individual IPv6
address is translated to a corresponding IPv4 address, and vice
versa, in cases where an algorithmic mapping is used. While specific
types of devices are used herein as examples, it is the
responsibility of the specification of such devices to reference this
document for algorithmic mapping of the addresses themselves.
Section 2 describes the prefixes and the format of "IPv4-embedded
IPv6 addresses", i.e., IPv6 addresses in which 32 bits contain an
IPv4 address. This format is common to both "IPv4-converted" and
"IPv4-translatable" IPv6 addresses. This section also defines the
algorithms for translating addresses, and the text representation of
IPv4-embedded IPv6 addresses.
Section 3 discusses the choice of prefixes, the conditions in which
they can be used, and the use of IPv4-embedded IPv6 addresses with
stateless and stateful translation.
Section 4 provides a summary of the discussions behind two specific
design decisions, the choice of a null suffix and the specific value
of the selected prefix.
Section 5 discusses security concerns.
In some scenarios, a dual-stack host will unnecessarily send its
traffic through an IPv6/IPv4 translator. This can be caused by the
host's default address selection algorithm [RFC3484], referrals, or
other reasons. Optimizing these scenarios for dual-stack hosts is
for future study.
1.1. Applicability Scope
This document is part of a series defining address translation
services. We understand that the address format could also be used
by other interconnection methods between IPv6 and IPv4, e.g., methods
based on encapsulation. If encapsulation methods are developed by
the IETF, we expect that their descriptions will document their
specific use of IPv4-embedded IPv6 addresses.

1.2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.3. Terminology
This document makes use of the following terms:
Address translator: any entity that has to derive an IPv4 address
from an IPv6 address or vice versa. This applies not only to
devices that do IPv4/IPv6 packet translation, but also to other
entities that manipulate addresses, such as name resolution
proxies (e.g., DNS64 [DNS64]) and possibly other types of
Application Layer Gateways (ALGs).
IPv4-converted IPv6 addresses: IPv6 addresses used to represent IPv4
nodes in an IPv6 network. They are a variant of IPv4-embedded
IPv6 addresses and follow the format described in Section 2.2.
IPv4-embedded IPv6 addresses: IPv6 addresses in which 32 bits
contain an IPv4 address. Their format is described in
Section 2.2.
IPv4/IPv6 translator: an entity that translates IPv4 packets to IPv6
packets, and vice versa. It may do "stateless" translation,
meaning that there is no per-flow state required, or "stateful"
translation, meaning that per-flow state is created when the first
packet in a flow is received.
IPv4-translatable IPv6 addresses: IPv6 addresses assigned to IPv6
nodes for use with stateless translation. They are a variant of
IPv4-embedded IPv6 addresses and follow the format described in
Section 2.2.
Network-Specific Prefix: an IPv6 prefix assigned by an organization
for use in algorithmic mapping. Options for the Network-Specific
Prefix are discussed in Sections 3.3 and 3.4.
Well-Known Prefix: the IPv6 prefix defined in this document for use
in an algorithmic mapping.

Bits 64 to 71 of the address are reserved for compatibility with the
host identifier format defined in the IPv6 addressing architecture
[RFC4291]. These bits MUST be set to zero. When using a /96
Network-Specific Prefix, the administrators MUST ensure that the bits
64 to 71 are set to zero. A simple way to achieve that is to
construct the /96 Network-Specific Prefix by picking a /64 prefix,
and then adding 4 octets set to zero.
The IPv4 address is encoded following the prefix, most significant
bits first. Depending of the prefix length, the 4 octets of the
address may be separated by the reserved octet "u", whose 8 bits MUST
be set to zero. In particular:
o When the prefix is 32 bits long, the IPv4 address is encoded in
positions 32 to 63.
o When the prefix is 40 bits long, 24 bits of the IPv4 address are
encoded in positions 40 to 63, with the remaining 8 bits in
position 72 to 79.
o When the prefix is 48 bits long, 16 bits of the IPv4 address are
encoded in positions 48 to 63, with the remaining 16 bits in
position 72 to 87.
o When the prefix is 56 bits long, 8 bits of the IPv4 address are
encoded in positions 56 to 63, with the remaining 24 bits in
position 72 to 95.
o When the prefix is 64 bits long, the IPv4 address is encoded in
positions 72 to 103.
o When the prefix is 96 bits long, the IPv4 address is encoded in
positions 96 to 127.
There are no remaining bits, and thus no suffix, if the prefix is 96
bits long. In the other cases, the remaining bits of the address
constitute the suffix. These bits are reserved for future extensions
and SHOULD be set to zero. Address translators who receive IPv4-
embedded IPv6 addresses where these bits are not zero SHOULD ignore
the bits' value and proceed as if the bits' value were zero. (Future
extensions may specify a different behavior.)

+-------------------+--------------+----------------------------+
| Well-Known Prefix | IPv4 address | IPv4-Embedded IPv6 address |
+-------------------+--------------+----------------------------+
| 64:ff9b::/96 | 192.0.2.33 | 64:ff9b::192.0.2.33 |
+-------------------+--------------+----------------------------+
Table 2: Text Representation of IPv4-Embedded IPv6 Addresses Using
the Well-Known Prefix
The Network-Specific Prefix examples in Table 1 are derived from the
IPv6 prefix reserved for documentation in [RFC3849]. The IPv4
address 192.0.2.33 is part of the subnet 192.0.2.0/24 reserved for
documentation in [RFC5735]. The representation of IPv6 addresses is
compatible with [RFC5952].
3. Deployment Guidelines
3.1. Restrictions on the Use of the Well-Known Prefix
The Well-Known Prefix MUST NOT be used to represent non-global IPv4
addresses, such as those defined in [RFC1918] or listed in Section 3
of [RFC5735]. Address translators MUST NOT translate packets in
which an address is composed of the Well-Known Prefix and a non-
global IPv4 address; they MUST drop these packets.
The Well-Known Prefix SHOULD NOT be used to construct IPv4-
translatable IPv6 addresses. The nodes served by IPv4-translatable
IPv6 addresses should be able to receive global IPv6 traffic bound to
their IPv4-translatable IPv6 address without incurring intermediate
protocol translation. This is only possible if the specific prefix
used to build the IPv4-translatable IPv6 addresses is advertised in
inter-domain routing, but the advertisement of more specific prefixes
derived from the Well-Known Prefix is not supported, as explained in
Section 3.2. Network-Specific Prefixes SHOULD be used in these
scenarios, as explained in Section 3.3.
The Well-Known Prefix MAY be used by organizations deploying
translation services, as explained in Section 3.4.
3.2. Impact on Inter-Domain Routing
The Well-Known Prefix MAY appear in inter-domain routing tables, if
service providers decide to provide IPv6-IPv4 interconnection
services to peers. Advertisement of the Well-Known Prefix SHOULD be
controlled either by upstream and/or downstream service providers
according to inter-domain routing policies, e.g., through

configuration of BGP [RFC4271]. Organizations that advertise the
Well-Known Prefix in inter-domain routing MUST be able to provide
IPv4/IPv6 translation service.
When the IPv4/IPv6 translation relies on the Well-Known Prefix, IPv4-
embedded IPv6 prefixes longer than the Well-Known Prefix MUST NOT be
advertised in BGP (especially External BGP) [RFC4271] because this
leads to importing the IPv4 routing table into the IPv6 one and
therefore introduces scalability issues to the global IPv6 routing
table. Administrators of BGP nodes SHOULD configure filters that
discard advertisements of embedded IPv6 prefixes longer than the
Well-Known Prefix.
When the IPv4/IPv6 translation service relies on Network-Specific
Prefixes, the IPv4-translatable IPv6 prefixes used in stateless
translation MUST be advertised with proper aggregation to the IPv6
Internet. Similarly, if translators are configured with multiple
Network-Specific Prefixes, these prefixes MUST be advertised to the
IPv6 Internet with proper aggregation.
3.3. Choice of Prefix for Stateless Translation Deployments
Organizations may deploy translation services using stateless
translation. In these deployments, internal IPv6 nodes are addressed
using IPv4-translatable IPv6 addresses, which enable them to be
accessed by IPv4 nodes. The addresses of these external IPv4 nodes
are then represented in IPv4-converted IPv6 addresses.
Organizations deploying stateless IPv4/IPv6 translation SHOULD assign
a Network-Specific Prefix to their IPv4/IPv6 translation service.
IPv4-translatable and IPv4-converted IPv6 addresses MUST be
constructed as specified in Section 2.2. IPv4-translatable IPv6
addresses MUST use the selected Network-Specific Prefix. Both IPv4-
translatable IPv6 addresses and IPv4-converted IPv6 addresses SHOULD
use the same prefix.
Using the same prefix ensures that IPv6 nodes internal to the
organization will use the most efficient paths to reach the nodes
served by IPv4-translatable IPv6 addresses. Specifically, if a node
learns the IPv4 address of a target internal node without knowing
that this target is in fact located behind the same translator that
the node also uses, translation rules will ensure that the IPv6
address constructed with the Network-Specific Prefix is the same as
the IPv4-translatable IPv6 address assigned to the target. Standard
routing preference (i.e., "most specific match wins") will then
ensure that the IPv6 packets are delivered directly, without
requiring that translators receive the packets and then return them
in the direction from which they came.

The intra-domain routing protocol must be able to deliver packets to
the nodes served by IPv4-translatable IPv6 addresses. This may
require routing on some or all of the embedded IPv4 address bits.
Security considerations detailed in Section 5 require that routers
check the validity of the IPv4-translatable IPv6 source addresses,
using some form of reverse path check.
The management of stateless address translation can be illustrated
with a small example:
We will consider an IPv6 network with the prefix 2001:db8:
122::/48. The network administrator has selected the Network-
Specific Prefix 2001:db8:122:344::/64 for managing stateless IPv4/
IPv6 translation. The IPv4-translatable address block for IPv4
subnet 192.0.2.0/24 is 2001:db8:122:344:c0:2::/96. In this
network, the host A is assigned the IPv4-translatable IPv6 address
2001:db8:122:344:c0:2:2100::, which corresponds to the IPv4
address 192.0.2.33. Host A's address is configured either
manually or through DHCPv6.
In this example, host A is not directly connected to the
translator, but instead to a link managed by a router R. The
router R is configured to forward to A the packets bound to 2001:
db8:122:344:c0:2:2100::. To receive these packets, R will
advertise reachability of the prefix 2001:db8:122:344:c0:2:2100::/
104 in the intra-domain routing protocol -- or perhaps a shorter
prefix if many hosts on link have IPv4-translatable IPv6 addresses
derived from the same IPv4 subnet. If a packet bound to
192.0.2.33 reaches the translator, the destination address will be
translated to 2001:db8:122:344:c0:2:2100::, and the packet will be
routed towards R and then to A.
Let's suppose now that a host B of the same domain learns the IPv4
address of A, maybe through an application-specific referral. If
B has translation-aware software, B can compose a destination
address by combining the Network-Specific Prefix 2001:db8:122:
344::/64 and the IPv4 address 192.0.2.33, resulting in the address
2001:db8:122:344:c0:2:2100::. The packet sent by B will be
forwarded towards R, and then to A, avoiding protocol translation.
Forwarding, and reverse path checks, are more efficient when
performed on the combination of the prefix and the IPv4 address. In
theory, routers are able to route on prefixes of any length, but in
practice there may be routers for which routing on prefixes larger
than 64 bits is slower. However, routing efficiency is not the only
consideration in the choice of a prefix length. Organizations also
need to consider the availability of prefixes, and the potential
impact of all-zero identifiers.

If a /32 prefix is used, all the routing bits are contained in the
top 64 bits of the IPv6 address, leading to excellent routing
properties. These prefixes may however be hard to obtain, and
allocation of a /32 to a small set of IPv4-translatable IPv6
addresses may be seen as wasteful. In addition, the /32 prefix and a
zero suffix lead to an all-zero interface identifier, which is an
issue that we discuss in Section 4.1.
Intermediate prefix lengths such as /40, /48, or /56 appear as
compromises. Only some of the IPv4 bits are part of the /64
prefixes. Reverse path checks, in particular, may have a limited
efficiency. Reverse path checks limited to the most significant bits
of the IPv4 address will reduce the possibility of spoofing external
IPv4 addresses, but would allow IPv6 nodes to spoof internal IPv4-
translatable IPv6 addresses.
We propose a compromise, based on using no more than 1/256th of an
organization's allocation of IPv6 addresses for the IPv4/IPv6
translation service. For example, if the organization is an Internet
Service Provider with an allocated IPv6 prefix /32 or shorter, the
ISP could dedicate a /40 prefix to the translation service. An end
site with a /48 allocation could dedicate a /56 prefix to the
translation service, or possibly a /96 prefix if all IPv4-
translatable IPv6 addresses are located on the same link.
The recommended prefix length is also a function of the deployment
scenario. The stateless translation can be used for Scenario 1,
Scenario 2, Scenario 5, and Scenario 6 defined in [v4v6-FRAMEWORK].
For different scenarios, the prefix length recommendations are:
o For Scenario 1 (an IPv6 network to the IPv4 Internet) and Scenario
2 (the IPv4 Internet to an IPv6 network), an ISP holding a /32
allocation SHOULD use a /40 prefix, and a site holding a /48
allocation SHOULD use a /56 prefix.
o For Scenario 5 (an IPv6 network to an IPv4 network) and Scenario 6
(an IPv4 network to an IPv6 network), the deployment SHOULD use a
/64 or a /96 prefix.
3.4. Choice of Prefix for Stateful Translation Deployments
Organizations may deploy translation services based on stateful
translation technology. An organization may decide to use either a
Network-Specific Prefix or the Well-Known Prefix for its stateful
IPv4/IPv6 translation service.

When these services are used, IPv6 nodes are addressed through
standard IPv6 addresses, while IPv4 nodes are represented by IPv4-
converted IPv6 addresses, as specified in Section 2.2.
The stateful nature of the translation creates a potential stability
issue when the organization deploys multiple translators. If several
translators use the same prefix, there is a risk that packets
belonging to the same connection may be routed to different
translators as the internal routing state changes. This issue can be
avoided either by assigning different prefixes to different
translators or by ensuring that all translators using the same prefix
coordinate their state.
Stateful translation can be used in scenarios defined in
[v4v6-FRAMEWORK]. The Well-Known Prefix SHOULD be used in these
scenarios, with two exceptions:
o In all scenarios, the translation MAY use a Network-Specific
Prefix, if deemed appropriate for management reasons.
o The Well-Known Prefix MUST NOT be used for Scenario 3 (the IPv6
Internet to an IPv4 network), as this would lead to using the
Well-Known Prefix with non-global IPv4 addresses. That means a
Network-Specific Prefix (for example, a /96 prefix) MUST be used
in that scenario.
4. Design Choices
The prefix that we have chosen reflects two design choices, the null
suffix and the specific value of the Well-Known Prefix. We provide
here a summary of the discussions leading to those two choices.
4.1. Choice of Suffix
The address format described in Section 2.2 recommends a zero suffix.
Before making this recommendation, we considered different options:
checksum neutrality, the encoding of a port range, and a value
different than 0.
In the case of stateless translation, there would be no need for the
translator to recompute a one's complement checksum if both the IPv4-
translatable and the IPv4-converted IPv6 addresses were constructed
in a "checksum-neutral" manner, that is, if the IPv6 addresses would
have the same one's complement checksum as the embedded IPv4 address.
In the case of stateful translation, checksum neutrality does not
eliminate checksum computation during translation, as only one of the
two addresses would be checksum neutral. We considered reserving 16
bits in the suffix to guarantee checksum neutrality, but declined

because it would not help with stateful translation and because
checksum neutrality can also be achieved by an appropriate choice of
the Network-Specific Prefix, i.e., selecting a prefix whose one's
complement checksum equals either 0 or 0xffff.
There have been proposals to complement stateless translation with a
port-range feature. Instead of mapping an IPv4 address to exactly
one IPv6 prefix, the options would allow several IPv6 nodes to share
an IPv4 address, with each node managing a different range of ports.
If a port range extension is needed, it could be defined later, using
bits currently reserved as null in the suffix.
When a /32 prefix is used, an all-zero suffix results in an all-zero
interface identifier. We understand the conflict with Section 2.6.1
of RFC4291, which specifies that all zeroes are used for the subnet-
router anycast address. However, in our specification, there is only
one node with an IPv4-translatable IPv6 address in the /64 subnet, so
the anycast semantic does not create confusion. We thus decided to
keep the null suffix for now. This issue does not exist for prefixes
larger than 32 bits, such as the /40, /56, /64, and /96 prefixes that
we recommend in Section 3.3.
4.2. Choice of the Well-Known Prefix
Before making our recommendation of the Well-Known Prefix, we were
faced with three choices:
o reuse the IPv4-mapped prefix, ::ffff:0:0/96, as specified in RFC
2765, Section 2.1;
o request IANA to allocate a /32 prefix, or
o request allocation of a new /96 prefix.
We weighted the pros and cons of these choices before settling on the
recommended /96 Well-Known Prefix.
The main advantage of the existing IPv4-mapped prefix is that it is
already defined. Reusing that prefix would require minimal
standardization efforts. However, being already defined is not just
an advantage, as there may be side effects of current
implementations. When presented with the IPv4-mapped prefix, current
versions of Windows and Mac OS generate IPv4 packets, but will not
send IPv6 packets. If we used the IPv4-mapped prefix, these nodes
would not be able to support translation without modification. This
will defeat the main purpose of the translation techniques. We thus
eliminated the first choice, i.e., decided to not reuse the IPv4-
mapped prefix, ::ffff:0:0/96.

A /32 prefix would have allowed the embedded IPv4 address to fit
within the top 64 bits of the IPv6 address. This would have
facilitated routing and load balancing when an organization deploys
several translators. However, such destination-address-based load
balancing may not be desirable. It is not compatible with Session
Traversal Utilities for NAT (STUN) [RFC5389] in the deployments
involving multiple stateful translators, each one having a different
pool of IPv4 addresses. STUN compatibility would only be achieved if
the translators managed the same pool of IPv4 addresses and were able
to coordinate their translation state, in which case there is no big
advantage to using a /32 prefix rather than a /96 prefix.
According to Section 2.2 of [RFC4291], in the legal textual
representations of IPv6 addresses, dotted decimal can only appear at
the end. The /96 prefix is compatible with that requirement. It
enables the dotted decimal notation without requiring an update to
[RFC4291]. This representation makes the address format easier to
use and the log files easier to read.
The prefix that we recommend has the particularity of being "checksum
neutral". The sum of the hexadecimal numbers "0064" and "ff9b" is
"ffff", i.e., a value equal to zero in one's complement arithmetic.
An IPv4-embedded IPv6 address constructed with this prefix will have
the same one's complement checksum as the embedded IPv4 address.
5. Security Considerations
5.1. Protection against Spoofing
IPv4/IPv6 translators can be modeled as special routers, are subject
to the same risks, and can implement the same mitigations. (The
discussion of generic threats to routers and their mitigations is
beyond the scope of this document.) There is, however, a particular
risk that directly derives from the practice of embedding IPv4
addresses in IPv6: address spoofing.
An attacker could use an IPv4-embedded IPv6 address as the source
address of malicious packets. After translation, the packets will
appear as IPv4 packets from the specified source, and the attacker
may be hard to track. If left without mitigation, the attack would
allow malicious IPv6 nodes to spoof arbitrary IPv4 addresses.
The mitigation is to implement reverse path checks and to verify
throughout the network that packets are coming from an authorized
location.

5.2. Secure Configuration
The prefixes used for address translation are used by IPv6 nodes to
send packets to IPv6/IPv4 translators. Attackers could attempt to
fool nodes, DNS gateways, and IPv4/IPv6 translators into using wrong
values for these parameters, resulting in network disruption, denial
of service, and possible information disclosure. To mitigate such
attacks, network administrators need to ensure that prefixes are
configured in a secure way.
The mechanisms for achieving secure configuration of prefixes are
beyond the scope of this document.
5.3. Firewall Configuration
Many firewalls and other security devices filter traffic based on
IPv4 addresses. Attackers could attempt to fool these firewalls by
sending IPv6 packets to or from IPv6 addresses that translate to the
filtered IPv4 addresses. If the attack is successful, traffic that
was previously blocked might be able to pass through the firewalls
disguised as IPv6 packets. In all such scenarios, administrators
should assure that packets that send to or from IPv4-embedded IPv6
addresses are subject to the same filtering as those directly sent to
or from the embedded IPv4 addresses.
The mechanisms for configuring firewalls and security devices to
achieve this filtering are beyond the scope of this document.
6. IANA Considerations
IANA has made the following changes in the "Internet Protocol Version
6 Address Space" registry located at http://www.iana.org.
OLD:
IPv6 Prefix Allocation Reference Note
----------- ---------------- ------------ ----------------
0000::/8 Reserved by IETF [RFC4291] [1][5]
NEW:
IPv6 Prefix Allocation Reference Note
----------- ---------------- ------------ ----------------
0000::/8 Reserved by IETF [RFC4291] [1][5][6]
[6] The "Well-Known Prefix" 64:ff9b::/96 used in an algorithmic
mapping between IPv4 to IPv6 addresses is defined out of the
0000::/8 address block, per RFC 6052.